This invention relates to a refrigeration control system, and more particularly to such a refrigeration control system particularly well suited for a cold drink or beverage (or other fluid) dispenser.
Cold drinks or beverages are oftentimes dispensed from a bulk source of the beverage via a dispensing valve an the beverage is refrigerated or otherwise chilled prior to the dispensing of such a cold drink into a cup. The beverage may either be a pre-mixed beverage (i.e., ready to drink from the bulk beverage source) or a post-mixed beverage (i.e., a concentrated syrup mixed with water, or, more usually, carbonated water).
In the dispensing of post-mixed carbonated soft drinks, particularly by high volume users, a severe refrigeration or chilling demand may, from time to time, be placed on the beverage dispensing system. Typically, in a post-mixed system, uncarbonated water from a city water line or the like is chilled by a refrigeration system and is carbonated prior to the chilled, carbonated water being mixed with the syrup to form the finished soft drink beverage. During periods of prolonged dispensing of beverages, particularly when the temperature of the water supply is relatively warm (such as in the summer time), the refrigeration system may not have sufficient refrigeration capability to chill the water down to a predetermined or desired temperature level. This insufficiently chilled water not only affects the temperature of the drink dispenser such that more ice is required to result in a cold beverage for the user, but the amount of carbonation in the drink may not be sufficient to yield a properly carbonated beverage. It will be appreciated that the solubility of carbon dioxide in water is highly influenced by the temperature of the water--the colder the water the more carbon dioxide may be dissolved therein.
Heretofore, beverage dispensers typically utilized an ice bank type water tank acting as a chilling reservoir for chilling the incoming water. In such ice bank reservoirs, a refrigeration coil was immersed in a water bath and the refrigeration coil was operated so as to freeze a quantity of ice around the coil with liquid water also remaining in contact with the ice. The beverage line was immersed in (i.e., in heat transfer relation with) the water in the ice bank water tank. As relatively warm water was circulated through the beverage line in the ice bath, it would be efficiently chilled by the ice bath water. As the water in the beverage line gave off heat to the water bath, a slight rise in temperature of the water bath would cause some ice to melt from the ice bank thus maintaining the water in the water tank at a desired low temperature (i.e., slightly above 32.degree. F.). Thus, as long as the mass of the ice bank would not be melted due to heat transfer to the water to be chilled running through the ice bank water tank, such ice bank systems were effective in chilling the beverage, even when the beverage was dispensed at a relatively high flow rate.
However, in many applications, such as fast food restaurants, movie theatres, and the like which have peak usage periods, the amount of beverage dispensed could overcome the capacity of such ice bank systems resulting in beverages being dispensed at higher than desirable temperatures.
In an effort to overcome this problem, the Cornelius Company of Anoka, Minn. developed a cold drink dispensing system as shown in U.S. Pat. No. 4,754,609, which, in addition to the ice bank water tank heretofore described, utilized a pre-cooling coil in heat transfer relation with the beverage (water) inlet line upstream from the ice bank so as to pre-chill the incoming beverage to lower the temperature of the beverage entering the ice bath to a predetermined maximum value. This pre-cooler coil did not utilize an ice bank and was intended to be in direct (i.e., conduction) heat transfer relationship with the incoming beverage and would be utilized only when the temperature of the incoming beverage exceeded a predetermined temperature level. Both the pre-cooler coil and the ice bank coil were supplied refrigerant from a common refrigeration system compressor and utilized conventional (i.e., mechanical) thermostatic expansion valves to regulate the flow of refrigerant through the pre-cooler coils and the ice bank coils and further utilized on/off solenoid valves to selectively open or block the flow of refrigerant through the pre-cooler coil.
However, it was found that operation of the above described two coil beverage dispensing system utilizing conventional mechanical thermostatic expansion valves was not entirely satisfactory add many desirable functions could not readily be accomplished without the addition of complicated controls and other solenoid valves which would increase the complexity and cost of the two coil beverage dispensing system.
Specifically, it was found that such two coil beverage dispensers controlled by mechanical thermostatic expansion valves experienced problems with the first or precooler coil freezing the water or beverage in heat transfer relation therewith when the flow of refrigerant through the first coil is blocked. Also, upon startup of the compressor, it was difficult to equalize the pressure in the two coils.
In addition to the above described two coil beverage dispensing system, reference should be made to the following U.S. Pat. Nos. which may be material to the examination of this invention: 3,557,743, 4,067,203, 4,459,819, 4,651,535, 4,467,613 and 4,685,309. These above-noted patents disclose various pulse modulated (i.e., open-closed) solenoid valves utilized as expansion valves where the ratio of open to closed time for the valve (duty cycle) was controlled by a suitable proportional or proportional-integration (also known as a sample and hold) control system which compared a system parameter (e.g., superheat) to a setpoint parameter and varied the duty cycle of the solenoid accordingly. However, within in the broader aspects of this invention, other types of modulated expansion valves, such as stepper motor actuated proportional valves or proportional (as opposed to open-closed) direct acting solenoid valves may be used.